- RT: access
n. ~ 1. Capability of being accessed or used. – 2. Computing · Ensuring timely and reliable access to and use of information (NIST, 2013, citing 44 USC 3542).
In computing, availability refers to the ability to access and use a system, and is often dependent on many components within a complex system, including software, hardware, and networking. Systems designed for high availability may require expensive redundancies.
- Gartner IT Glossary (†298 s.v. availability): The assurance that an enterprise’s IT infrastructure has suitable recoverability and protection from system failures, natural disasters or malicious attacks.
- CNSS-4009 (†730 p. 6): The property of being accessible and useable upon demand by an authorized entity. (†1685)
- Cohen 2008 (†652 p. 176): Availability is typically measured as percentage of down time per unit time. (†1476)
- Han 2011 (†644 p.204): Amazon S3 and Google AppEngine are claimed to be highly available and highly reliable. . . . The cloud computing providers have huge advantages in offering high availability to minimize hardware failure, natural disasters, network failure, and human errors, while the locally managed server and storage approach has to be invested a lot to reduce these risks. (†1450)
- ISACA Glossary (†743 s.v. availability): Ensuring timely and reliable access to and use of information. (†1762)
- Loveland, et al. 2008 (†643 p.591): The more available a computer system is, the more value it can provide to its users. When a computer system is designed for high availability (HA), its downtime over a given interval is guaranteed to be smaller than a specified limit. Achieving such an availability objective often involves the use of redundant physical resources, which, for some users, may prohibitively increase its cost. Also, the software configuration activity and management tasks required to assemble and coordinate all elements of the HA solution and its monitoring infrastructure into a single, well-functioning unit can be daunting. The resulting complexity can further inhibit the adoption of HA solutions. (†1448)
- Loveland, et al. 2008 (†643 p.602): Availability requirements vary with the application – they range from noncritical to continuous. The requirements for increased availability are balanced against the associated costs. Virtualization offers the potential to decrease these costs and thus to make higher levels of availability possible for a given budget. (†1449)
- NIST 2013 (†734 p. B-2): Ensuring timely and reliable access to and use of information. [44 U.S.C., Sec. 3542] (†1841)
- NIST FIPS 199 2004 (†735 p. 2): A loss of availability is the disruption of access to or use of information or an information system. (†1686)
- Pace 2014 (†664 p.11-12): Reliability also has multiple subcomponents. For example, we can separate the probability of losing data (reliability) from the probability that data is temporarily not accessible (availability). As discussed later in the reliability chapters, there are data-encoding techniques that are resilient to hardware failures only in terms of data loss but not in terms of availability. (†1519)
- Russell, et al. 2010 (†653 p.191): Many people confuse or interchange the concepts of system reliability and availability. These concepts are most frequently discussed in a hardware or equipment context, but also have applicability to software. Most often associated with component or system failure, reliability is a measure of the likelihood that a system or process will perform its designed function for a specified period of time. Availability is a relative measure of the extent that a system can perform its designed function (Bhagwan et al. 2003). As a relative measure, availability includes metrics such as delays, congestion, and loading. (†1491)
- Russell, et al. 2010 (†653 p.194): Typically the availability of components, software, and systems is given in terms of its likelihood of being available; not the inverse (the likelihood it is not available). In reliability practice there is a concept called five nines that refers to uptime (availability). If a system is said to have five nines reliability, it is operational 99.999% of the time. This likelihood translates to being inoperable 31.5 s per year. (†1492)
- Stitilis and Malinauskaite 2013 (†693 p.401): Providing availability means ensuring timely and reliable access to personal data. Each end-user of cloud services wants to be sure that, if necessary, he or she will have easy access to their data. It should also be noted that the principle of availability in practice is associated with specific risks. One special threat to availability in the cloud is accidental loss of network connectivity between the end-user and the service provider or of server performance caused by malicious actions, such as (Distributed) Denial of Service (DoS) attacks. Other availability risks include accidental hardware failures both on the network and in the cloud processing and data storage systems, power failures and other infrastructure problems. Therefore, the end-users, before starting to use cloud services, shall check whether the cloud service provider has adopted reasonable measures to settle the risks. (†1579)